Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides, and particulate matter. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Many different techniques have been applied to exhaust systems to clean the exhaust gas before it passes to atmosphere.
A typical exhaust system comprises (1) a NOx adsorber catalyst (or “NOx trap”) to adsorb NOx from the exhaust gas, (2) an oxidation catalyst to oxidize hydrocarbons that were not combusted in the internal combustion engine and to convert CO to CO2, and (3) a carbon soot filter to remove particulates from the exhaust gas before release to atmosphere. Various combinations of these systems have been disclosed.
U.S. Appl. Pub. No. 2011/0005200 teaches a catalyst system that simultaneously removes ammonia and enhances net NOx conversion by placing an ammonia-selective catalytic reduction (“NH3-SCR”) catalyst formulation downstream of a lean NOx trap. The NH3-SCR catalyst is taught to adsorb the ammonia that is generated during the rich pulses in the lean NOx trap. The stored ammonia then reacts with the NOx emitted from the upstream lean NOx trap, which increases NOx conversion rate while depleting the stored ammonia.
U.S. Appl. Pub. No. 2011/0014099 teaches a catalytically active particulate filter useful for removing nitrogen oxides and particulates from exhaust gases of internal combustion engines operated predominantly under lean conditions. The particulate filter comprises a filter body, a platinum group metal oxidation catalyst active coating, and a copper compound located in a second coating.
PCT Intl. Appl. WO 2008/075111 teaches an exhaust system including (1) a NOx adsorber catalyst (“NAC”), (2) a catalyzed soot filter, (3) means for enriching the exhaust gas intermittently during normal lean running operation in order to remove sulfate (as H2S) adsorbed on the NAC, and (4) a compound located downstream of at least some of the NAC, which is effective to remove and/or convert at least some of the generated H2S in the enriched exhaust gas. The H2S removal compound may be located at various positions in the exhaust system, including (a) between the NAC and the catalyzed soot filter, (b) on a substrate downstream of the NAC, (c) on the catalyzed soot filter, (d) between the catalyzed soot filter and the exhaust system exit, and (e) various combinations of (a)-(d).
PCT Intl. Appl. WO 2010/114873 discloses emissions treatment systems that contain ammonia-generating catalysts such as NOx storage reduction or lean NOx trap (“LNT”) catalysts in combination with selective catalytic reduction (“SCR”) catalysts, and methods for their use with lean burn engines. The LNT and SCR catalysts can be used in a variety of configurations. For instance the LNT and SCR catalysts can be located (1) each on a separate substrate brick, (2) in a front zone and a rear zone of a single substrate brick, or (3) on a diesel particulate filter that is downstream of the SCR, such that each component can be on its own substrate or the SCR can be coated on a front zone of the diesel particulate filter or along the entire length of the diesel particulate filter.
PCT Intl. Appl. WO 2011/023332 discloses an exhaust-gas aftertreatment system having a wall-flow filter as component (1) and, downstream thereof, a flow-through monolith as component (2). Components (1) and (2) have at least one storage function for the same compound, selected from the group comprising SOx, NOx, NH3, O2, hydrocarbons, and H2S, characterized in that the storage capacity in the component (2) is to be designed such that the breakthrough signal downstream of component (2) has the highest gradient of the concentration curve resulting at the respective termination criterion taken into consideration for the exhaust-gas compound while as little as possible of the storage material is used.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new exhaust system and catalyzed substrate that provide enhanced cleaning of the exhaust gases from internal combustion engines.